Stand-by instruments are autonomous onboard instruments generating and displaying flight information that is essential to the pilot of an aircraft in the event of failure of primary onboard instruments. This flight information, generally obtained with lower precision than that of the primary onboard instruments, essentially relates to the speed, altitude and attitude of the aircraft. In order to guarantee the autonomy of the stand-by instruments with respect to the primary onboard instruments, the stand-by instruments must have their own sensors in order to generate and display the speed, altitude and attitude of the aircraft. In particular, the stand-by instruments usually comprise a static pressure sensor, a total pressure sensor and an inertial measurement unit.
The static and total pressure sensors are respectively connected to a static pressure port and a total pressure port located on the skin of the aircraft. The static pressure makes it possible to determine the altitude of the aircraft. The difference between the total pressure and the static pressure makes it possible to determine the speed of the aircraft with respect to the air.
The inertial measurement unit comprises for example three gyrometers and two or three accelerometers. The gyrometers measure the speeds of rotation of the sensor's reference system, in this case a system of axes related to the stand-by instrument, with respect to an inertial reference system. By integration of the rotation speeds, it is possible to know the orientation of the stand-by instrument with respect to the inertial reference system and therefore, knowing the orientation of the stand-by instrument with respect to the aircraft and the orientation of the local geographic reference system with respect to the inertial reference system, it is possible to know the orientation of the aircraft with respect to the local geographic reference system. The orientation of the aircraft with respect to the local geographic reference system, called the attitude of the aircraft, is referenced with respect to a roll axis, a pitch axis and a yaw axis and the movements about these axes are called the roll, the pitch and the yaw respectively. The accelerometers measure non-gravitational forces applied to the aircraft, from which are derived accelerations of translation of the reference system of the sensor with respect to the inertial reference system. The combination of the gyrometers and the accelerometers allows a precise determination of the attitude of the aircraft, the data provided by the accelerometers being used in preference to the data provided by the gyrometers in the static or quasi-static flight phases, and the data provided by the gyrometers being used in preference to the data provided by the accelerometers during dynamic flight phases.
During the powering up of an aircraft and in particular of a stand-by instrument, the inertial measurement unit of the stand-by instrument must be initialized in order to provide attitude information which is as reliable as possible during the flight. This initialization comprises an alignment phase notably consisting in estimating a drift of each gyrometer, that is to say a rotation speed measured by the gyrometer in question in the absence of any movement of the latter. As gyrometers are electronic sensors, their drift can differ between two switch-ons of the inertial measurement unit, to such a degree as to make any measurement carried out by these gyrometers and therefore any attitude displayed by the stand-by instrument useless. It is therefore necessary to determine the drift of the gyrometers at each switch-on. However, the switching on of the stand-by instrument can take place whilst the aircraft is standing on a stable platform, for example a runway, on an unstable platform, for example an oil rig platform, an aircraft carrier or a helicopter carrier, or even when it is in flight, after a more or less brief cut-off of the electrical power supply of the stand-by instrument. When the aircraft is standing on an unstable platform or when it is in flight, the alignment of the inertial measurement unit takes into account measurements of drifts due, not only to the intrinsic drifts of the inertial measurement unit, but also of the movements of the aircraft. In particular, the alignment can take into account movements due to wave motion when the aircraft is standing on an oil rig platform and movements due to atmospheric disturbances when it is in flight. Consequently, the alignment can be falsified by the movement of the aircraft.
In order to ensure the correct alignment of the inertial measurement unit, it is known to check for the presence or absence of movements of the inertial measurement unit by means of accelerometers of the inertial measurement unit. Throughout the whole of the duration of the alignment, the accelerometers measure the non-gravitational forces of the inertial measurement unit with respect to the inertial reference system. In the case of movement of the inertial measurement unit during the alignment, measured by the accelerometers, the stand-by instrument, at the end of the alignment, invalidates the determination of the drift of each gyrometer, displays a message indicating to the pilot the detection of movement and requesting the pilot to restart the alignment either by switching off the stand-by instrument and then switching it on again, or by pressing a button on the front face of the stand-by instrument. This restarting of the alignment is imperative insofar as the availability of the stand-by instrument, and therefore the alignment of the inertial measurement unit, is a necessary condition for the authorization of the take-off of the aircraft. Such a solution has several disadvantages. A first disadvantage is reaching the end of the alignment in order to indicate the detection of a movement during the alignment. It is therefore only at the end of the alignment of the gyrometers that the pilot knows of the invalidation of the alignment and can restart it. Consequently, the time elapsed between the detection of movement and the end of the alignment is wasted. A second disadvantage is the loss of the estimation of the drifts carried out between the start of the alignment and the detection of a movement. At the end of the invalidated alignment, the whole alignment procedure is restarted, the estimated drift risking having being falsified by the movement of the inertial measurement unit. If the alignment is restarted by a hardware reset, that is to say by switching off the stand-by instrument and then switching it on again, there is an even greater risk that the drift of the gyrometers will change, making the previous determination of the drifts obsolete. A third disadvantage is the impossibility of being able to carry out an alignment in certain situations. This can notably arise when the aircraft is started up on a moving platform. In most cases, the movement of the platform, for example due to the wave motion of the sea, cannot be prevented. The aircraft must therefore wait for the movement to stop, in this case for the wave motion to become calm, in order to be able to take off. Such an immobilization is incontestably harmful to the profitability of aircraft. Similarly, when the aircraft is in flight, the stand-by instrument risks being subjected to movements due to the piloting of the aircraft and to disturbances of the air surrounding the aircraft. Even though the pilot can limit the movements due to the piloting of the aircraft, he cannot prevent movements due to air disturbances. The alignment of the inertial measurement unit cannot therefore be carried out.